U.S. patent number 9,054,382 [Application Number 13/328,834] was granted by the patent office on 2015-06-09 for binder for secondary battery exhibiting excellent adhesion force.
This patent grant is currently assigned to LG CHEM, LTD.. The grantee listed for this patent is MinAh Kang, Bhom Ri Kim, Ok Sun Kim, Young-Min Kim. Invention is credited to MinAh Kang, Bhom Ri Kim, Ok Sun Kim, Young-Min Kim.
United States Patent |
9,054,382 |
Kang , et al. |
June 9, 2015 |
Binder for secondary battery exhibiting excellent adhesion
force
Abstract
Provided is a binder for secondary battery electrodes comprising
polymer particles obtained by polymerizing (a) a (meth)acrylic acid
ester monomer; (b) at least one monomer selected from the group
consisting of an acrylate monomer, a vinyl monomer and a nitrile
monomer; and (c) a (meth)acrylamide monomer and an unsaturated
monocarbonic acid monomer, with two or more cross-linking agents
with different molecular weights. Based on the combination of
specific components, the binder basically improves stability of an
electrode in the process of fabricating the electrode, thus
providing secondary batteries with superior cycle properties.
Inventors: |
Kang; MinAh (Daejeon,
KR), Kim; Young-Min (Daejeon, KR), Kim;
Bhom Ri (Daejeon, KR), Kim; Ok Sun (Daejeon,
KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
Kang; MinAh
Kim; Young-Min
Kim; Bhom Ri
Kim; Ok Sun |
Daejeon
Daejeon
Daejeon
Daejeon |
N/A
N/A
N/A
N/A |
KR
KR
KR
KR |
|
|
Assignee: |
LG CHEM, LTD. (Seoul,
KR)
|
Family
ID: |
44507325 |
Appl.
No.: |
13/328,834 |
Filed: |
December 16, 2011 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20120183848 A1 |
Jul 19, 2012 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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PCT/KR2010/008876 |
Dec 11, 2010 |
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Foreign Application Priority Data
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Feb 26, 2010 [KR] |
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10-2010-0017560 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
4/0404 (20130101); H01M 4/622 (20130101); H01M
10/0525 (20130101); H01M 2220/20 (20130101); H01M
2220/30 (20130101) |
Current International
Class: |
H01M
4/62 (20060101); H01M 4/13 (20100101); C08F
301/00 (20060101); H01M 4/04 (20060101); H01M
10/0525 (20100101) |
Field of
Search: |
;429/217 ;252/182.1
;526/209,216,307.6 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1720628 |
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Jan 2006 |
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CN |
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1249882 |
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Oct 2002 |
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EP |
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2003-268053 |
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Sep 2003 |
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JP |
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10-0491026 |
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May 2005 |
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KR |
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10-2007-0001186 |
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Jan 2007 |
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KR |
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10-0711975 |
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May 2007 |
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KR |
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10-2008-0062966 |
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Jul 2008 |
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KR |
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10-2009-0017939 |
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Feb 2009 |
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KR |
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10-2009-0019630 |
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Feb 2009 |
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KR |
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2009019630 |
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Feb 2009 |
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KR |
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Other References
Derwent Abstract for Kim et al., KR 2009-019630 A. cited by
examiner .
Machine translation for Kim et al., KR 2009-019630 A. cited by
examiner .
International Search Report, dated Jul. 29, 2011 in
PCT/KR2010/008876. cited by applicant.
|
Primary Examiner: Enin-Okut; Edu E
Attorney, Agent or Firm: Birch, Stewart, Kolasch &
Birch, LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a Continuation of PCT/KR2010/008876 filed on
Dec. 11, 2010, which claims priority of Application No.
10-2010-0017560 filed in Republic of Korea on Feb. 26, 2010, all of
which are hereby expressly incorporated by reference into the
present application.
Claims
The invention claimed is:
1. A binder for secondary battery electrodes comprising polymer
particles obtained by polymerizing (a) a (meth)acrylic acid ester
monomer; (b) at least one monomer selected from the group
consisting of an acrylate monomer, a vinyl monomer and a nitrile
monomer; and (c) a (meth)acrylamide monomer and a unsaturated
monocarbonic acid monomer, with two or more cross-linking agents
with different molecular weights, wherein the cross-linking agents
comprise the cross-linking agent having a low molecular weight not
lower than 50 and lower than 250 and the cross-linking agent having
a high molecular weight not lower than 250 and lower than 20,000,
and are present at a mix ratio of cross-linking agent having low
molecular weight:cross-linking agent having high molecular weight,
on a weight basis from 1:0.1 to 1:20.
2. The binder according to claim 1, wherein, (a) monomer is present
in an amount of 10 to 98% by weight, (b) monomer is present in an
amount of 1 to 60% by weight, and (c) monomer is present in an
amount of 1 to 20% by weight, based on the total weight of the
binder.
3. The binder according to claim 1, wherein the (meth)acrylic acid
ester monomer is at least one monomer selected from the group
consisting of methyl acrylate, ethyl acrylate, propyl acrylate,
isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-amyl
acrylate, isoamyl acrylate, n-ethyl hexyl acrylate, 2-ethyl hexyl
acrylate, 2-hydroxy ethyl acrylate, methyl methacrylate, ethyl
methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl
methacrylate, isobutyl methacrylate, n-amyl methacrylate, isoamyl
methacrylate, n-hexyl methacrylate, n-ethyl hexyl methacrylate,
2-ethyl hexyl methacrylate, hydroxyethyl methacrylate and
hydroxypropyl methacrylate.
4. The binder according to claim 1, wherein the acrylate monomer is
selected from the group consisting of methacryloxy ethylethylene
urea, .beta.-carboxy ethylacrylate, aliphatic monoacrylate,
dipropylene diacrylate, ditrimethylolpropane tetraacrylate,
hydroxyethyl acrylate, dipentaerythritol hexaacrylate,
pentaerythritol triacrylate, pentaerythritol tetraacrylate, lauryl
acrylate, cetyl acrylate, stearyl acrylate, lauryl methacrylate,
cetyl methacrylate and stearyl methacrylate.
5. The binder according to claim 1, wherein the nitrile monomer is
at least one selected from the group consisting of succinonitrile,
sebaconitrile, fluoronitrile, chloronitrile, acrylonitrile,
methacrylonitrile and the like.
6. The binder according to claim 1, wherein the vinyl monomer is at
least one selected from the group consisting of styrene,
.alpha.-methylstyrene, .beta.-methylstyrene, p-t-butylstyrene,
divinyl benzene and mixtures thereof.
7. The binder according to claim 1, wherein the (meth)acrylamide
monomer is at least one selected from the group consisting of
acrylamide, n-methylol acrylamide, n-butoxymethyl acrylamide,
methacrylamide and mixtures thereof.
8. The binder according to claim 1, wherein the unsaturated
monocarbonic acid monomer is at least one selected from maleic
acid, fumaric acid, methacrylic acid, acrylic acid, glutaconic
acid, itaconic acid, tetrahydrophthalic acid, crotonic acid,
isocrotonic acid, nadic acid or a mixture thereof.
9. The binder according to claim 1, wherein a ratio of the
(meth)acrylamide monomer to the unsaturated monocarbonic acid
monomer is preferably 1:20 to 1:2, on the basis of weight.
10. The binder according to claim 1, wherein, of the cross-linking
agents, the cross-linking agent having a low molecular weight is a
(meth)acrylate compound or an amine compound which has two or more
terminal double bonds.
11. The binder according to claim 10, wherein, the (meth)acrylate
compound is at least one compound selected from the group
consisting of ethylene glycol dimethacrylate, 1,3-butane diol
dimethacrylate, 1,6-hexane diol dimethacrylate, aryl methacrylate
(AMA), and triallyl isocyanurate (TAIC).
12. The binder according to claim 10, wherein the amine compound is
at least one compound selected from the group consisting of
triallyl amine (TAA) and diallyl amine (DAA).
13. The binder according to claim 1, wherein, of the cross-linking
agents, the cross-linking agent having a high molecular weight is a
(meth)acrylate compound which has two or more terminal double bonds
with an oxyalkylene group.
14. The binder according to claim 13, wherein the (meth)acrylate
compound is at least one compound selected from the group
consisting of polyethylene glycol diacrylate, polypropylene glycol
diacrylate and polybutylene glycol diacrylate.
15. The binder according to claim 13, wherein the cross-linking
agents are present in an amount of 0.1 to 10% by weight, based on
the total weight of the binder.
16. A slurry for electrodes comprising: (a) the binder for
electrodes according to claim 1; and (b) an electrode active
material capable of intercalating and de-intercalating lithium.
17. An electrode for secondary batteries, in which the slurry for
electrodes according to claim 16 is applied to a current
collector.
18. A lithium secondary battery comprising the electrode for
secondary batteries according to claim 17.
Description
TECHNICAL FIELD
The present invention relates to a binder for secondary battery
electrodes. More specifically, the present invention relates to a
binder for secondary battery electrodes comprising polymer
particles obtained by polymerizing a (meth)acrylic acid ester
monomer; at least one monomer selected from the group consisting of
an acrylate monomer, a vinyl monomer and a nitrile monomer; and a
(meth)acrylamide monomer and an unsaturated monocarbonic acid
monomer, with two or more cross-linking agents with different
molecular weights.
BACKGROUND ART
Rapidly increasing use of fossil fuels has led to an increase in
demand for use of alternative or clean energy. In light of such
trends, generation and storage of electricity using electrochemical
reaction are a very active area of research.
In recent years, representative examples of electrochemical devices
using electrochemical energy are secondary batteries, and
application range thereof continues to expand.
Recently, technological development and increased demand associated
with portable equipment such as portable computers, cellular phones
and cameras have brought about an increase in the demand for
secondary batteries as energy sources. Among these secondary
batteries, lithium secondary batteries having high energy density
and operating electric potential, long lifespan and low
self-discharge have been actively researched and are commercially
available and widely used.
In addition, increased interest in environmental issues has led to
a great deal of research into electric vehicles, hybrid electric
vehicles or the like as alternatives to vehicles using fossil fuels
such as gasoline vehicles and diesel vehicles. These electric
vehicles and hybrid electric vehicles generally use nickel-metal
hydride secondary batteries as power sources. However, a great deal
of study associated with lithium secondary batteries with high
energy density and discharge voltage is currently underway and some
are commercially available.
Conventional typical lithium secondary batteries use graphite as an
anode active material. Lithium ions of a cathode are repeatedly
intercalated into and de-intercalated from the anode to realize
charge and discharge. The theoretical capacity of batteries may
vary depending upon the type of the electrode active material, but
generally cause deterioration in charge and discharge capacity in
the course of the cycle life of the battery.
The primary reason behind such phenomenon is that separation
between an electrode active material or separation between the
electrode active material and a current collector due to volume
variation of the electrode, as batteries are charged and
discharged, results in insufficient realization of function of the
active material. In addition, in the process of intercalation and
de-intercalation, lithium ions intercalated into the anode cannot
be sufficiently de-intercalated and active sites of the anode are
thus decreased. For this reason, charge/discharge capacity and
lifespan of batteries may decrease as the batteries are cycled.
In particular, in order to improve discharge capacity, in the case
where natural graphite having a theoretical discharge capacity of
372 mAh/g is used in combination with a material such as silicon,
tin or silicon-tin alloys having high discharge capacity, volume
expansion of the material considerably increases, in the course of
charging and discharging, thus causing isolation of the anode
material from the electrode material. As a result, battery capacity
disadvantageously rapidly decreases over repeated cycling.
Accordingly, there is an increasing demand in the art for binder
and electrode materials which can prevent separation between the
electrode active material, or between the electrode active material
and the current collector upon fabrication of electrodes via strong
adhesion and can control volume expansion of electrode active
materials upon repeated charging/discharging via strong physical
properties, thus improving structural stability of electrodes and
thus performance of batteries.
Polyvinylidene difluoride (PVdF), a conventional solvent-based
binder, does not satisfy these requirements. Recently, a method for
preparing a binder, in which styrene-butadiene rubber (SBR) is
polymerized in an aqueous system to produce emulsion particles and
the emulsion particles are mixed with a neutralizing agent, or the
like, is used and is commercially available. Such a binder is
advantageous in that it is environmentally friendly and reduces use
of the binder and thereby increasing battery capacity. However,
this binder exhibits improved adhesion maintenance due to the
elasticity of rubber, but has no great effect on adhesion
force.
Accordingly, there is an increasing need for development of binders
which improves cycle properties of batteries, contributes to
structural stability of electrodes and exhibits superior adhesion
strength.
DISCLOSURE
Technical Problem
Therefore, the present invention has been made to solve the above
and other technical problems that have yet to be resolved.
As a result of a variety of extensive and intensive studies and
experiments to solve the problems as described above, the inventors
of the present invention have developed a binder for secondary
battery electrodes comprising polymer particles obtained by
polymerizing a specific combination of monomers with two or more
cross-linking agents with different molecular weights, as described
below, and confirmed that the use of this binder contributes to
improvement in cycle properties of batteries and adhesion strength.
The present invention was completed based on this discovery.
Technical Solution
Accordingly, the binder for secondary battery electrodes according
to the present invention comprises polymer particles obtained by
polymerizing (a) a (meth)acrylic acid ester monomer; (b) at least
one monomer selected from the group consisting of an acrylate
monomer, a vinyl monomer and a nitrile monomer; and (c) a
(meth)acrylamide monomer and an unsaturated monocarbonic acid
monomer, with two or more cross-linking agents with different
molecular weights.
In accordance with the binder of the present invention, a specific
combination of monomers provides superior adhesion strength and
improved adhesion maintenance and a cross-linking agent having a
low molecular weight improves binder physical properties, thus
improving cycle characteristics, and a cross-linking agent having a
high molecular weight improves flexibility of the binder, thus
improving adhesion strength.
In a preferred embodiment, based on the total weight of the binder,
(a) monomer may be present in an amount of 10 to 98% by weight, (b)
monomer may be present in an amount of 1 to 60% by weight, and (c)
monomer may be present in an amount of 1 to 20% by weight. More
preferably, (a) monomer is present in an amount of 20 to 95% by
weight, (b) monomer is present in an amount of 3 to 50% by weight,
and (c) monomer is present in an amount of 2 to 15% by weight.
These content ranges may be suitably varied depending on the
characteristics of the monomers and physical properties of the
binder.
For example, the (meth)acrylic acid ester monomer, as (a) monomer,
may be at least one monomer selected from the group consisting of
methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl
acrylate, n-butyl acrylate, isobutyl acrylate, n-amyl acrylate,
isoamyl acrylate, n-hexyl acrylate, 2-ethyl hexyl acrylate,
2-hydroxy ethyl acrylate, methyl methacrylate, ethyl methacrylate,
propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate,
isobutyl methacrylate, n-amyl methacrylate, isoamyl methacrylate,
n-hexyl methacrylate, n-ethyl hexyl methacrylate, 2-ethyl hexyl
methacrylate, hydroxyl ethyl methacrylate and hydroxyl propyl
methacrylate.
For example, the acrylate monomer, as (b) monomer, may be selected
from the group consisting of methacryloxy ethylethylene urea,
.beta.-carboxy ethylacrylate, aliphatic monoacrylate, dipropylene
diacrylate, ditrimethylolpropane tetraacrylate, hydroxyethyl
acrylate, dipentaerythritol hexaacrylate,
pentaerythritoltriacrylate, pentaerythritoltetraacrylate, lauryl
acrylate, cetyl acrylate, stearyl acrylate, lauryl methacrylate,
cetyl methacrylate and stearyl methacrylate.
The vinyl monomer, as (b) monomer, may be at least one selected
from the group consisting of styrene, .alpha.-methylstyrene,
.beta.-methylstyrene, p-t-butylstyrene, divinyl benzene and
mixtures thereof.
The nitrile monomer, as (b) monomer, may be at least one selected
from the group consisting of succinonitrile, sebaconitrile,
fluoronitrile, chloronitrile, acrylonitrile, methacrylonitrile and
the like. More preferably, the nitrile monomer is at least one
selected from the group consisting of acrylonitrile,
methacrylonitrile and mixtures thereof.
The (meth)acrylamide monomer, as (c) monomer, may be at least one
selected from the group consisting of acrylamide, n-methylol
acrylamide, n-butoxymethyl acrylamide, methacrylamide and mixtures
thereof.
The unsaturated monocarbonic acid monomer, as (c) monomer, may be
at least one selected from maleic acid, fumaric acid, methacrylic
acid, acrylic acid, glutaconic acid, itaconic acid,
tetrahydrophthalic acid, crotonic acid, isocrotonic acid, nadic
acid or a mixture thereof.
In the (c) monomer, a ratio of the (meth)acrylamide monomer to the
unsaturated monocarbonic acid monomer is preferably 1:20 to 1:2,
more preferably, 1:10 to 1:3, on the basis of weight.
As described above, the binder of the present invention comprises,
in addition to the monomers, two or more cross-linking agents with
different molecular weights polymerized with the monomers.
Of these, the cross-linking agent having a low molecular weight is
preferably a (meth)acrylate compound or an amine compound which has
two or more terminal double bonds and a molecular weight not higher
than 50 and lower than 250.
For example, the (meth)acrylate compound may be at least one
compound selected from the group consisting of ethylene glycol
dimethacrylate, 1,3-butane diol dimethacrylate, 1,6-hexane diol
dimethacrylate, aryl methacrylate (AMA), and triallyl isocyanurate
(TAIC).
For example, the amine compound may be at least one compound
selected from the group consisting of triallyl amine (TAA) and
diallyl amine (DAA).
The cross-linking agent having a high molecular weight is
preferably a (meth)acrylate compound which has two or more terminal
double bonds with an oxyalkylene group and has a molecular weight
not lower than 250 and lower than 20,000.
For example, the (meth)acrylate compound may be at least one
compound selected from the group consisting of polyethylene glycol
diacrylate, polypropylene glycol diacrylate and polybutylene glycol
diacrylate.
Meanwhile, the cross-linking agents are present at a mix ratio of
1:0.1 to 1:20 (cross-linking agent having low molecular
weight:cross-linking agent having large molecular weight, on the
basis of weight) and is present in an amount of 0.1 to 10% by
weight, based on the total weight of the binder. When the content
of the cross-linking agent having a low molecular weight is
excessively low, improvement of cycle characteristics cannot be
obtained, and when the content of the cross-linking agent having a
high molecular weight is excessively low, improvement in binder
flexibility cannot be obtained. In addition, when the content of
the cross-linking agents is excessively low, volume variation of
the electrode upon charge/discharge cannot be controlled and
capacity maintenance is thus low, and when the content of the
cross-linking agents is excessively high, high adhesion strength
cannot be obtained.
The binder according to the present invention may be prepared by
emulsion polymerization using the monomers and the cross-linking
agents. The polymerization temperature and polymerization period
may be suitably determined depending on the polymerization method
or polymerization initiator employed, and for example, the
polymerization temperature may be from about 50.degree. C. to
200.degree. C. and the polymerization period may be from about 1 to
about 20 hours.
Examples of the emulsifying agent used for emulsion polymerization
include oleic acid, stearic acid, lauric acid, fatty acid salts
such as sodium or potassium salts of mixed fatty acids and general
anionic emulsifying agents such as rosin acid. Preferably, a
reactive emulsifying agent to improve stability of latex may be
added. The emulsifying agent may be used alone or in combination
thereof.
In addition, the polymerization initiator for emulsion
polymerization may be an inorganic or organic peroxide and examples
thereof include water-soluble initiators including potassium
persulfate, sodium persulfate and ammonium persulfate, and
oil-soluble initiators including cumene hydroperoxide and benzoyl
peroxide. In addition, an activating agent to promote initiation
reaction of peroxide may be further included with the
polymerization initiators. For example, the activating agent may be
at least one selected from the group consisting of sodium
formaldehyde sulfoxylate, sodium ethylenediaminetetraacetate,
ferrous sulfate, dextrose and combinations thereof.
The present invention provides a slurry comprising the
aforementioned electrode binder and an electrode active material
capable of intercalating and de-intercalating lithium.
The slurry may contain a predetermined solvent such as water and
NMP. The electrode active material will be described in more detail
below.
An electrode may be fabricated by applying the slurry to a current
collector, followed by drying and rolling.
Accordingly, the present invention provides an electrode for
secondary batteries in which the slurry is applied to the current
collector. The electrode for secondary batteries may be a cathode
or an anode.
For example, the cathode is fabricated by applying a mixture
consisting of a cathode active material, a conductive material and
a binder to a cathode current collector, followed by drying. The
anode is fabricated by applying a mixture consisting of an anode
active material, a conductive material and a binder to an anode
current collector, followed by drying. In some cases, the anode may
comprise no conductive material.
The electrode active material is a material causing electrochemical
reaction in the electrode and is divided into a cathode active
material and an anode active material depending on the type of
electrode.
The cathode active material is lithium transition metal oxide which
includes two or more transition metals, and examples thereof
include, but are not limited to, layered compounds such as lithium
cobalt oxide (LiCoO.sub.2) or lithium nickel oxide (LiNiO.sub.2)
substituted with one or more transition metals; lithium manganese
oxide substituted with one or more transition metals; lithium
nickel oxide represented by the formula of
LiNi.sub.1-yM.sub.yO.sub.2 (in which M=Co, Mn, Al, Cu, Fe, Mg, B,
Cr, Zn or Ga and includes one or more elements among the elements,
0.01.ltoreq.y.ltoreq.0.7); lithium nickel cobalt manganese
composite oxides represented by
Li.sub.1+zNi.sub.bMn.sub.cCo.sub.1-(b+c+d)M.sub.dO.sub.(2-e)A.sub.e
such as Li.sub.1+zNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2 or
Li.sub.1+zNi.sub.0.4Mn.sub.0.4Co.sub.0.2O.sub.2 (in which
-0.5.ltoreq.z.ltoreq.0.5, 0.1.ltoreq.b.ltoreq.0.8,
0.1.ltoreq.c.ltoreq.0.8, 0.ltoreq.d.ltoreq.0.2,
0.ltoreq.e.ltoreq.0.2, b+c+d<1, M=Al, Mg, Cr, Ti, Si or Y, A=F,
P or Cl); and olivine lithium metal phosphate represented by the
formula Li.sub.1+xM.sub.1-yM'.sub.yPO.sub.4-zX.sub.z (in which
M=transition metal, preferably Fe, Mn, Co or Ni, M'=Al, Mg or Ti,
X=F, S or N, -0.5.ltoreq.x.ltoreq.+0.5, 0.ltoreq.y.ltoreq.0.5, and
0.ltoreq.z.ltoreq.0.1).
Examples of the anode active material include carbon and graphite
materials such as natural graphite, artificial graphite, expanded
graphite, carbon fiber, hard carbon, carbon black, carbon
nanotubes, perylene, activated carbon; metals alloyable with
lithium, such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt and
Ti and compounds containing these elements; composites of carbon
and graphite materials with a metal and a compound thereof; and
lithium-containing nitrides. Of these, a carbon-based active
material, a silicon-based active material, a tin-based active
material, or a silicon-carbon-based active material are more
preferred. The material may be used alone or in combination of two
or more thereof.
The conductive material serves to further improve the electrode
active material and is commonly added in an amount of 0.01 to 30%
by weight, based on the total weight of the electrode mix. Any
conductive material may be used without particular limitation so
long as it has suitable conductivity without causing adverse
chemical changes in the fabricated secondary battery. Examples of
conductive materials that can be used in the present invention
include conductive materials, including graphite such as natural or
artificial graphite; carbon blacks such as carbon black, acetylene
black, Ketjen black, channel black, furnace black, lamp black and
thermal black; carbon derivatives such as carbon nanotubes or
fullerene; conductive fibers such as carbon fibers and metallic
fibers; metallic powders such as carbon fluoride powder, aluminum
powder and nickel powder; conductive whiskers such as zinc oxide
and potassium titanate; conductive metal oxides such as titanium
oxide; and conductive materials such as polyphenylene derivatives.
The current collector in the electrode is a material causing
electrochemical reaction and is divided into a cathode current
collector and an anode current collector depending on the type of
electrode.
The cathode current collector is generally fabricated to have a
thickness of 3 to 500 .mu.m. There is no particular limit to the
cathode current collector, so long as it has suitable conductivity
without causing adverse chemical changes in the fabricated battery.
As examples of the cathode current collector, mention may be made
of stainless steel, aluminum, nickel, titanium, sintered carbon,
and aluminum or stainless steel surface-treated with carbon,
nickel, titanium, silver or the like.
The anode current collector is generally fabricated to have a
thickness of 3 to 500 .mu.m. There is no particular limit to the
anode current collector, so long as it has suitable conductivity
without causing adverse chemical changes in the fabricated battery.
As examples of the anode current collector, mention may be made of
copper, stainless steel, aluminum, nickel, titanium, sintered
carbon, and copper or stainless steel surface-treated with carbon,
nickel, titanium, silver, or the like, and aluminum-cadmium
alloys.
These current collectors include fine irregularities on the surface
thereof so as to enhance adhesion to electrode active materials. In
addition, the current collectors may be used in various forms
including films, sheets, foils, nets, porous structures, foams and
non-woven fabrics.
The mixture (electrode mix) of an electrode active material, a
conductive material and a binder may further comprise at least one
selected from the group consisting of a viscosity controller and a
filler.
The viscosity controller controls the viscosity of the electrode
mix so as to facilitate mixing of the electrode mix and application
thereof to the current collector and may be added in an amount of
30% by weight, based on the total weight of the electrode mix.
Examples of the viscosity controller include, but are not limited
to, carboxymethylcellulose, polyacrylic acid and polyvinylidene
fluoride. If necessary, the solvent may also serve as a viscosity
controller.
The filler is a component used to inhibit expansion of the
electrode. There is no particular limit to the filler, so long as
it does not cause adverse chemical changes in the fabricated
battery and is a fibrous material. As examples of the filler, there
may be used olefin polymers such as polyethylene and polypropylene;
and fibrous materials such as glass fibers and carbon fibers.
The present invention also provides a lithium secondary battery
comprising the electrode for secondary batteries.
The lithium secondary battery generally further comprises a
separator and a lithium salt-containing non-aqueous electrolyte, in
addition to the electrodes.
The separator is interposed between the cathode and anode. As the
separator, an insulating thin film having high ion permeability and
mechanical strength is used. The separator typically has a pore
diameter of 0.01 to 10 .mu.m and a thickness of 5 to 300 .mu.m. As
the separator, sheets or non-woven fabrics made of an olefin
polymer such as polypropylene and/or glass fibers or polyethylene,
which have chemical resistance and hydrophobicity, are used. When a
solid electrolyte such as a polymer is employed as the electrolyte,
the solid electrolyte may also serve as both the separator and
electrolyte.
The lithium salt-containing, non-aqueous electrolyte is composed of
a non-aqueous electrolyte and a lithium salt.
As the non-aqueous electrolytic solution that can be used in the
present invention, for example, mention may be made of non-protic
organic solvents such as N-methyl-2-pyrollidinone, propylene
carbonate, ethylene carbonate, butylene carbonate, dimethyl
carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxy
ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran,
dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide,
dioxolane, acetonitrile, nitromethane, methyl formate, methyl
acetate, phosphoric acid triester, trimethoxy methane, dioxolane
derivatives, sulfolane, methyl sulfolane,
1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,
tetrahydrofuran derivatives, ether, methyl propionate and ethyl
propionate.
The lithium salt is a material that is readily soluble in the
above-mentioned non-aqueous electrolyte and may include, for
example, LiCl, LiBr, LiI, LiClO.sub.4, LiBF.sub.4,
LiB.sub.10Cl.sub.10, LiPF.sub.6, LiCF.sub.3SO.sub.3,
LiCF.sub.3CO.sub.2, LiAsF.sub.6, LiSbF.sub.6, LiAlCl.sub.4,
CH.sub.3SO.sub.3Li, CF.sub.3SO.sub.3Li,
(CF.sub.3SO.sub.2).sub.2NLi, chloroborane lithium, lower aliphatic
carboxylic acid lithium, lithium tetraphenyl borate and imides.
An organic solid electrolyte or an inorganic solid electrolyte may
be used, if necessary.
As examples of the organic solid electrolyte utilized in the
present invention, mention may be made of polyethylene derivatives,
polyethylene oxide derivatives, polypropylene oxide derivatives,
phosphoric acid ester polymers, poly agitation lysine, polyester
sulfide, polyvinyl alcohols, polyvinylidene fluoride, and polymers
containing ionic dissociation groups.
As examples of the inorganic solid electrolyte utilized in the
present invention, mention may be made of nitrides, halides and
sulfates of lithium such as Li.sub.3N, LiI, Li.sub.5NI.sub.2,
Li.sub.3N--LiI--LiOH, LiSiO.sub.4, LiSiO.sub.4--LiI--LiOH,
Li.sub.2SiS.sub.3, Li.sub.4SiO.sub.4, Li.sub.4SiO.sub.4--LiI--LiOH
and Li.sub.3PO.sub.4--Li.sub.2S--SiS.sub.2.
Additionally, in order to improve charge/discharge characteristics
and flame retardancy, for example, pyridine, triethylphosphite,
triethanolamine, cyclic ether, ethylenediamine, n-glyme,
hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone
imine dyes, N-substituted oxazolidinone, N,N-substituted
imidazolidine, ethylene glycol dialkyl ether, ammonium salts,
pyrrole, 2-methoxy ethanol, aluminum trichloride or the like may be
added to the non-aqueous electrolyte. If necessary, in order to
impart incombustibility, the non-aqueous electrolyte may further
comprise halogen-containing solvents such as carbon tetrachloride
and ethylene trifluoride. Further, in order to improve
high-temperature storage characteristics, the non-aqueous
electrolyte may additionally include carbon dioxide gas,
fluoro-ethylene carbonate (FEC), propene sultone (PRS) or
fluoro-ethylene carbonate (FEC).
The secondary batteries according to the present invention may be
used as a power source of electric vehicles (EV) or hybrid electric
vehicles (REV) which require long cycle properties, high rate
properties and the like.
Advantageous Effects
As apparent from the foregoing, the binder for secondary battery
electrodes according to the present invention comprises polymer
particles obtained by polymerizing a specific combination of
monomers with two or more cross-linking agents with different
molecular weights and thus provides improved cycle properties and
high adhesion strength.
BEST MODE
Now, the present invention will be described in more detail with
reference to the following Examples. These examples are provided
only to illustrate the present invention and should not be
construed as limiting the scope and spirit of the present
invention.
EXAMPLE 1
Butyl acrylate (60 g), styrene (30 g), acrylic acid (5 g) and acryl
amide (1 g) as monomers were added to water containing ethylene
glycol dimethacrylate (0.5 g) and polyethylene glycol
dimethacrylate (0.5 g) having a molecular weight of 400, as
cross-linking agents, sodium lauryl sulfate as an emulsifying agent
and potassium persulfate as a polymerization initiator, and these
ingredients were mixed and polymerized at 70.degree. C. for about 5
hours. A binder for secondary battery electrodes containing polymer
particles obtained by polymerizing the monomers with the
cross-linking agents was prepared through polymerization.
EXAMPLE 2
A binder for secondary battery electrodes was prepared in the same
manner as in Example 1 except that 2-ethylhexylacrylate was used as
a monomer, instead of butylacrylate.
EXAMPLE 3
A binder for secondary battery electrodes was prepared in the same
manner as in Example 1 except that acrylonitrile was used as a
monomer, instead of styrene.
COMPARATIVE EXAMPLE 1
A binder for secondary battery electrodes was prepared in the same
manner as in Example 1 except that acrylic acid (5 g) was used as a
monomer.
COMPARATIVE EXAMPLE 2
A binder for secondary battery electrodes was prepared in the same
manner as in Example 1 except that acrylonitrile (1 g) was not used
as a monomer.
COMPARATIVE EXAMPLE 3
A binder for secondary battery electrodes was prepared in the same
manner as in Example 1 except that acrylic acid (5 g) and
acrylamide (1 g) were not used as monomers.
EXPERIMENTAL EXAMPLE 1
Adhesion Strength Test
In the case where the polymer binder according to the present
invention was used as an anode binder for lithium secondary
batteries, adhesion strength between an electrode active material
and a current collector was measured.
First, for the binders of Examples 1 to 3 and the binders of
Comparative Examples 1 to 3, an active material, a viscosity
controller and the binder were mixed in a ratio of 97:1:2 to
prepare a slurry and the slurry was coated on an Al foil to
fabricate an electrode.
The surface of the electrode thus fabricated was cut and fixed to a
glass slide and 180 degree peel strength was measured, while the
current collector was peeled off. The results thus obtained are
shown in Table 1. Evaluation was based on an average of five or
more peel strengths.
TABLE-US-00001 TABLE 1 Adhesion strength (g) Ex. 1 41 Ex. 2 44 Ex.
3 40 Comp. Ex. 1 33 Comp. Ex. 2 35 Comp. Ex. 3 32
As can be seen from Table 1 above, electrodes employing the binders
of Examples 1 to 3 according to the present invention exhibited
considerably high adhesion strength, as compared to electrodes
employing the binders of Comparative Examples 1 to 3. It can be
seen that adhesion strength can be greatly improved by adding the
unsaturated monocarbonic monomer and the (meth)acrylamide
monomer.
Although the preferred embodiments of the present invention have
been disclosed for illustrative purposes, those skilled in the art
will appreciate that various modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as disclosed in the accompanying
claims.
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